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                    <h1>AVFoundation学习笔记七 异步加载属性及媒体元数据</h1>
                    
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                            Posted by Cyrus on
                        
                        
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                <p>AVAsset有多种方法和属性，比如时长、创建日期和元数据等，同时还包含一些用于获取和使用曲目集合的方法。AVAsset使用延迟载入资源的属性（懒加载）,直到请求才载入。这样可以更快地创建AVAsset对象（没有载入相关媒体/元数据的时间）。这样会产生一些问题，那就是首次访问某些属性时，如asset的duration(总时长），如MP3文件没有在头文件中设置TLEN标签（用于记录duration值）,则需要解析整个文件来确定时长。如果发生在主线程，就会阻塞主线程。所以，AVAsset/AVAssetTrack/ AVMetadataItem等类都实现了AVAsynchronousKeyValueLoading协议，以实现异步查询属性的功能。</p>
<h3 id="AVAsynchronousKeyValueLoading-协议"><a href="#AVAsynchronousKeyValueLoading-协议" class="headerlink" title="AVAsynchronousKeyValueLoading 协议"></a>AVAsynchronousKeyValueLoading 协议</h3><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">//查询给定属性是否会导致程序卡顿，AVKeyValueStatusLoaded:不会卡顿，反之卡顿</span><br><span class="line">- (AVKeyValueStatus)statusOfValueForKey:(NSString *)key error:(NSError * _Nullable * _Nullable)outError;</span><br><span class="line"></span><br><span class="line">//异步加载属性</span><br><span class="line">- (void)loadValuesAsynchronouslyForKeys:(NSArray&lt;NSString *&gt; *)keys completionHandler:(nullable void (^)(void))handler;</span><br></pre></td></tr></table></figure>
<p>代码示例<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br></pre></td><td class="code"><pre><span class="line">NSURL *assetURL = [[NSBundle mainBundle] URLForResource:@&quot;xxx&quot; withExtension:@&quot;xxx&quot;];</span><br><span class="line">    AVAsset *asset = [AVAsset assetWithURL:assetURL];</span><br><span class="line">    </span><br><span class="line">    //1、设置要查询的属性（数组）</span><br><span class="line">    NSArray *keys = @[@&quot;tracks&quot;];</span><br><span class="line">    [asset loadValuesAsynchronouslyForKeys:keys completionHandler:^&#123;</span><br><span class="line">        //判断相关属性的状态并进行对应处理</span><br><span class="line">        AVKeyValueStatus status = [asset statusOfValueForKey:@&quot;tracks&quot; error:nil];</span><br><span class="line">        switch (status) &#123;</span><br><span class="line">            case AVKeyValueStatusLoaded:</span><br><span class="line">                //continue processing</span><br><span class="line">                break;</span><br><span class="line">            case AVKeyValueStatusFailed:</span><br><span class="line">                //handle failure with error</span><br><span class="line">                break;</span><br><span class="line">            case AVKeyValueStatusCancelled:</span><br><span class="line">                //handle explicit cancellation</span><br><span class="line">                break;</span><br><span class="line">            default:</span><br><span class="line">                //handle all other case</span><br><span class="line">                break;</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;];</span><br></pre></td></tr></table></figure></p>
<p>注意：loadValuesAsynchronouslyForKeys:completionHandler: 可以一次查询多个属性，但completionHandle只会回调一次，且所查询的属性调用statusOfValueForKey：的返回值不一定相同。</p>


                
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